Gravity waves in the mesosphere observed with the middle and upper atmosphere radar

Abstract

We have observed wind motions from 60 to 90 km altitudes with the middle and upper atmosphere (MU) radar during daylight hours (0800-1600 LT) from October 13 to 31, 1986. Gravity waves with fairly sinusoidal vertical structure were evident on 16 days of the 19 days of observations. They were characterized by a typical vertical wavelength of 5-15 km and intrinsic periods centered at about 8.6 hours. The propagation velocity of these waves was determined using the linear gravity wave theory. All of the waves propagated downward and had an equatorward component of the meridional propagation. The median direction of horizontal propagation was slightly east of south, and the mean horizontal phase speed was 35.3 m/s. The vertical wave number spectra of horizontal components of mesoscale wind fluctuations agree well with the theoretical spectrum of saturated gravity waves. At frequencies from 1 x 10 -5 to about 4 x 10 -5 c/s frequency spectra of vertical and radial wind components had logarithmic slopes of 1/3 and -5/3, respectively, which agree fairly well with a model gravity wave spectrum. The effects of Doppler shifting on frequency spectra are most obviously recognized at large frequencies near the Brunt Vfiisfilfi frequency. We have also determined the upward flux of horizontal momentum induced by waves with periods from 10 min to 8 hours and further esffmated the westward and northward body force of 5.1 and 4.0 m/s/d, respectively. ity waves have been determined by using a crosscorrelation analysis of wind fields measured by radars in regions horizontally separated by several tens to a few hundred kilometers [Vincent and Reid, 1983; Meek et al., 1985a; Yarnarnoto et al., 1986; Manson and Meek, 1988]. For gravity waves with periods less than 1 hour, Vincent and Reid determined typical values of about 70 km and 70 m/s for the zonal horizontal wavelength and zonal phase velocity at 80-94 km. Meek et al. determined horizontal scales ranging from 68 km to 119 km and phase velocity from 20 to 93 m/s for waves with intrinsic periods of 10-100 min. From meteor radar observations at 90-100 km Yamamoto et al. have determined ---25 m/s and ---360 km for mean values of zonal phase velocity and wavelength for waves with periods from 2 to 8 hours, and they also found that the westward propagating waves are dominant in winter. By using a linear dispersion relation Hirota and Niki [1985] studied characteristics of gravity waves from rocketsonde observations collected at 30-60 km altitudes.

Seasonal and interannual variations of gravity wave activity in the low-latitude mesosphere and lower thermosphere over Tirunelveli (8.7° N, 77.8° E)

“…Buhler et al (2003) studied the equatorward propagation of inertia gravity waves due to steady and intermittent wave forcing and showed that even wave sources outside the equatorial region can robustly produce potential energy spectra that peak at the equator. Tsuda et al (1990) found gravity wave motions of period centered around 8.6 h in the MU radar observations having equatorward component of the meridional propagation.…”

Statistical characteristics of AGW wave packet propagation in the lower atmosphere observed by the MU radar

“…Radar observations by Vincent and Fritts (1987) and rocket observations by Eckermann and Vincent (1989) in the Southern Hemisphere revealed some difference in the distribution of propagation directions between summer and winter. Tsuda et al (1990) found that the preferential direction of the dominant gravity wave component at 60-90 km altitude in October 1986 over Shigaraki Japan was south-southeastward. Gavrilov et al (1997) analyzed the internal gravity waves (IGWs) in the altitude range of 65-85 km over Shigaraki Japan by statistical methods; and decided that the propagation direction was mainly eastward in summer and westward in winter, they also decided that most of IGWs have momentum fluxes directed to the northeast in winter and to the east in summer.…”

The nonlinear effects on the characteristics of gravity wave packets: dispersion and polarization relations

“…Assuming the initial background is an isothermal atmosphere in hydrostatic equilibrium that is horizontally homogeneous; radar measurements (Tsuda et al, 1990) have shown that mean zonal winds in the range of 60±90 km altitude are 0±40 m s )1 , so the initial values of horizontal and vertical wind velocities in this study are assumed to be 10 and 0 m s )1 , respectively. Thus we have…”

“…However, any of the groundbased remote sensing facilities can only provide information of wave frequency and vertical wave number (Vincent and Fritts, 1987;Vincent 1994). The horizontal wave number is usually estimated by using the correlation analysis, or derived from the polarization and dispersion relations (Meek et al, 1985;Yamamoto et al, 1986;Manson, 1990;Tsuda et al, 1990). Therefore it is specially signi®cant to examine quantitatively the validity of dispersion and polarization relations based essentially on the linear gravity-wave theory in the nonlinear region.…”